It is known in the plastic extrusion art that the end use of the finished product may dictate that predetermined portions of the product have thickness variations therein. Illustratively, in the production of corrugated thermoplastic tubing or tile for drainage and water flow systems, structural integrity in the finished profile may dictate that the wall thickness of the ribs be relatively greater than the wall thickness of the valleys therebetween. Accomplishment of this objective under manufacturing conditions is often difficult, wherein there exists steady state conditions of constant melt output from an extruder or constant speed melt pump, and a corrugator operating at constant speed. Such conditions frequently produce corrugated tubing in which the plastic forming the rib is relatively thinner than the plastic of the inner diameter or valley, presumably because the plastic in the latter location has been stretched to the larger diameter.
Attempts have been made to overcome this problem by varying the proportions of the rib and valley, both longitudinally and diametrally, and by periodic variations in the operating speed of the corrugator. Although the intent of this pursuit was to permit more plastic to be directed into the ribbed portion of the profile, under normal manufacturing conditions this result has not been universally achieved.